For increasing riding comfort and for minimum occupant fatigue, a vehicle seat backrest must support the lumbar area of the seat occupant's back. But it is recognized, that the construction of universally satisfactory permanent backrest structures can not be effected because of differences in individual body sizes and proportions.
In view of the above, it has become popular to equip a vehicle seat with a so-called "adjustable lumbar supporter" held in the backrest of the seat. However, such an adjustable lumbar supporter conventionally used still has a limitation in setting a plurality of positions. In fact, the adjustment of the supporter is limited to either forward-rearward positioning or upward-downward positioning. More specifically, the positioning of the lumbar supporter is made by moving it monodirectionally. Thus, the most desired and effective positioning of the supporter for properly supporting the lumbar area of the seated occupant has not been obtained.